Abstract

The distribution of the mean oceanic oxygen concentration results from a balance betweenventilation and consumption and, in particular, reveals extended oxygen minimumzones (OMZ) in the eastern tropical Pacific and Atlantic at intermediate depth(300m - 700m). It has been recently shown that OMZs expand in size and are subject to asignificant oxygen decrease, where the OMZ in the Tropical North East Atlantic (TNEA)holds the most significant and largest oxygen trend.

This study analyzes hydrographic and velocity data from shipboard and moored observationswhich were acquired along the 23°W section cutting meridionally through theTNEA OMZ, in order to (i) quantify regional differences in the oxygen variability, (ii)identify the role of two mixing processes (mesoscale stirring and diapycnal mixing) inthe production of oxygen variance based on the extended Osborn-Cox model and (iii)estimate the role of mesoscale eddies in the meridional ventilation of the TNEA OMZ.

Enhanced oxygen variability is found at the boundaries of the OMZ which is predominantlygenerated by mesoscale stirring along isopycnals and diapycnal mixing acrossisopycnals. South of the OMZ core (which is located at about 11°N), mesoscale stirringdominates the generation of oxygen variance, whereas above the OMZ core within thedeep oxycline (located at about 300m depth) both processes, mesoscale stirring and diapycnalmixing, are found to be of similar importance.

Meridional eddy fluxes of oxygen are estimated by using (i) a diffusive flux parameterizationbased on a lateral eddy diffusion coefficient and the mean isopycnal oxygengradient, and (ii) a correlation method based on velocity and oxygen time series frommoored observations. At the mooring positions 5°N, 23°W and 8°N, 23°W, the resultsof both methods are in good agreement in the depth range of the OMZ core, yielding anorthward oxygen flux from the well-ventilated equatorial region toward the OMZ core.

The divergence of the meridional oxygen flux, as obtained from the diffusive flux parameterization,yields an eddy-driven meridional oxygen supply from the south into theOMZ of about 2.4 μmol kg−1 yr−1 at its core depth. Above the OMZ core, mesoscale eddiesact to redistribute low-oxygen and high-oxygen waters associated with latitudinallyalternating westward and eastward currents. Locally, absolute values of the eddy-drivenmeridional oxygen supply are greater than 10 μmol kg−1 yr−1 which is likely balanced bymean zonal advection.

Combining the above results with recent studies about oxygen consumption, diapycnaloxygen supply and oxygen tendency, a refined oxygen budget for the TNEA OMZis derived. The eddy-driven meridional oxygen supply accounts for more than 50% ofthe supply required to balance the estimated oxygen consumption. The oxygen tendencyin the TNEA OMZ, as given by the multidecadal oxygen decline, is at maximum slightlyabove the OMZ core and represents a substantial imbalance of the oxygen budget reachingup to 20% of the magnitude of the eddy-driven oxygen supply.

Oxygen data from moored observations was acquired with optode oxygen sensors.To achieve a high quality of these measurements, an in situ calibration procedure is proposedwhich yields an average measurement error of 4.6 μmol kg−1 with 95% confidence(evaluated for water masses in the upper 1000m of the Tropical Atlantic in combinationwith onboard lab calibrations against zero oxygen). On the one hand, this calibrationprocedure yields calibration errors that are worse by roughly a factor of 2 in comparisonto sophisticated laboratory calibrations, but on the other hand this simple method is notaffected by severe sensor drifts that are frequently observed at some time before or afterthe field deployment. In case of profiling systems (shown here for a CTD/O2 cast), atime constant correction improves the quality of the measured oxygen profile which is ofparticular interest for the application in gliders or floats.